Presses for compacting electrically conductive compositions



y 8, 1968 F. E. BALL ET AL 3,386,017

PRESSES FOR COMPACTING ELECTRICALLY CONDUCTIVE COMPOSITIONS Filed June 12, 1964 4 Sheets-Sheet 1 FIG. I.

May 28, 1968 5 L ET AL 3,386,017

PRESSES FOR COMPACTING ELECTRICALLY CONDUCTIVE COMPOSITIONS I Filed June 12, 1964 I 4 Sheets-Sheet 3 FIG. 2.

May 28, 1968 F. E. BALL ET AL 3,386,017

PRESSES FOR COMPACTING ELECTRICAL'LY CONDUCTIVE COMPOSIT IONS Filed June 12, 1964 4 Sheets-Shea: 3

FIG. 3.

. POWER CONSTANT CURRENT SUPPLY IGNITER CONTROL PRESS I UNIT /|5 cowummonv cmcun AMP. TACHO FIG. 4.

May 28, 1968 I E, BALL ET AL 3,386,017

PRESSES FOR COMPACTING ELECTRICALLY'CONDUCTIVE COMPOSITIONS Filed June 12, 1964 v 4 Sheets-Sheet 4 6O R4 R5 IGNITER \/\/V\/\/\ TO AMP l FIG. 5.

6O\ R4 R5 I v MMMV RI IGNITER v 59 '\/\/VV\/\ United States Patent 3,386,017 PRESSES FOR COMPACTING ELECTRICALLY CONDUCTIVE COMPOSITIONS Francis Edward Ball, London, and John William Martin,

Dartford, England, assignors to National Research Development Corporation, London, England Filed June 12, 1964, Ser. No. 374,651 Claims priority, application Great Britain, June 13, 1963,

7 Claims. (Cl. 318-18) ABSTRACT OF THE DISCLOSURE A press for pressing compacts of electrically conductive compositions includes a servosystem for controlling the press ram in accordance with the electrical resistance of the compact. The servosystem includes a comparator for comparing a reference resistance with the varying resistance of the compact during pressing. The comparator output controls a motor which powers drive means for the ram such that the ram speed decreases as the resistance of the compact approaches the reference resistance.

This invention relates to presses for compacting electrically conductive compositions.

Such compositions, which consist of mixtures of electrically conductive material, frequently graphite, with non-conductive material, exhibit resistance characteristics which vary with the degree of compaction. The variation of resistance with load is by no means regular and similar bodies of pressed material (hereinafter referred to as compacts) of the same composition, pressed to a constant final load, have been found to have, for some purposes, an excessive spread in resistance values. This has been the case in some instances where the non-conductive component is explosive, as in some initiating compositions, since the sensitivity of the initiator vares wth resistance and a wide spread in sensitivity is obviously undesirable.

This variation in resistance characteristics can be greatly reduced by pressing the composition until the resistance of the compact reaches a predetermined value. Variation in the final resistance of the compact is not entirely eliminated since, in general, the resistance increases by an inconsistent amount on release of the load after compaction. The spread in resistance of compacts and hence, in the case of initiating compositions, in sensitivity of initiators is, however, reduced to a much more acceptable level.

The invention therefore comprises a press for compacting electrically conductive compositions in which press the movement of the ram is controlled by a servosystem which, in turn, is controlled by the resistance of the compact being pressed, whereby the pressing operation is terminated when the resistance of the compact reaches a predetermined value.

The press may be of any suitable type having, for example, a mechanically, hydraulically or pneumatically driven ram. A suitable control circuit may include a reference resistance of the said predetermined value, connected in series with the compact and with a source of current, the voltages across the reference resistance and the compact respectively, being fed through a differential amplifier to an electric motor which controls the operation of the ram. The driving current to the motor is thus proportional to the difference between the voltages across the compact and the reference resistance, which difference decreases from a maximum value when the composition is uncompressed, to zero when the compact resistance is equal to that of the reference. At this point the motor stops and terminates the pressing operation.

The load may be released by short-circuiting the compact. This has the same effect as a reduction of compact resistance below that of the reference and causes the motor to run in the reverse direction to release the load. Since, owing to the nature of the composition, the decrease in resistance with increasing load may be very irregular, it is usually desirable to apply to the motor, through the amplifier, a stabilising feed back voltage derived from a tachometer driven by the motor.

In an alternative circuit, the reference resistance and compact constitute adjacent arms of a resistance bridge whose other arms are resistances of equal value. The opposite vertices between compact and reference and between the equal resistances are connected to an amplifier whose output controls the motor, a current source being connected across the other pair of vertices. In this case also the motor driving current decreases from a maximum value, when the composition is uncompressed, to zero when the compact resistance and reference resistances are equal.

On form of press in accordance with the invention and suitable for pressing igniter or initiator fillings will now be more particularly described by way of example only with reference to the accompanying drawings in which:

FIGURE 1 is a side elevation of the press,

FIGURE 2 is an axial section of a mould for use with the press,

FIGURE 3 is a detail of the electrical connections to the mould,

FIGURE 4 is a block diagram of the servosystem for controlling the press, and

FIGURES 5 and 6 are two alternative comparator circuits for use in the servosystem.

The ram 1 of the press is actuated through a beam 2, to an intermediate point of which the ram 1 is connected, the said beam 2 being pivoted near one end to the upper end of a pillar 3 which extends upwardly from a suitable base 4. The ram 1 is loaded by a pull exerted on the end of the beam 2 remote from the pillar 3 by means of a cord 5 which is wound onto a spindle or drum of a winch 6 driven through a gearbox 7, by an electric motor 8 which constitutes the final stage of a servosystem by means of which the press is controlled. The cord 5 passes over a pulley 9 which is attached to one end of a springbalance 10, the other end of which is attached to the beam 2, the end of the cord 5 being anchored to a lug 11 fixed to a casing 12 in which the press is enclosed. This spring balance 10 serves both as a resilient element and as an indicator of the load applied. The beam 2 and ram 1 arrangement provides a suitable leverage factor between the load thus applied and the pressing load of the ram 1. In operation a suitable, thick casing or mould 13, carrying an igniter is positioned below the ram 1, the casing being so conducted (as shown in FIG. 2) as to provide electrical contacts with the igniter filling which is connected into the circuit later to be described. The beam is biased toward the load released position by a counterweight 14 adjustably mounted on the end of the beam 2 remote from that to which the load is applied. Electrical connections to the mould 13, the thence to the igniter being pressed, are shown in more detail in FIGURES 2 and 3 and will of course be subject to variations in detail according to the construction of the igniter. In addition to driving the winch 6 the motor 8 drives a tachometer 15 which is used, as hereinafter described, to supply a stabilizing feed back to an amplifier which constitutes part of the servosystem.

The enclosing casing 12 of the press is, in part, provided as a safety measure when pressing explosive fillings and when a door 16 is closed, the ram 1 and mould 13 of the press are almost totally enclosed. For further ensuring safety two microswitches 17 and 17a are provided which are operated by the door 16. One of these is arranged to cut off the power supply to the motor armature and to a control unit of the servosystem on opening the door 16 so that pressing cannot continue when the door is open. The other switch is arranged to short circuit the igniter on opening the door, to prevent its detonation, for example, by pick up of stray current.

Two further microswitches 18, 19 are included in the press as shown in FIG. 1. The first of these, 18, is operated by the ram 1, when raised to the limit of its return stroke, to cut the power supply to the motor field by way of a comparator circuit in the servosystem in order to prevent the motor over running when the load is released and rewinding the cord 5 in the reverse direction on the winch. The other switch 19 is attached to the spring balance and is operated by the balance pointer 20, at a predetermined load, to short circuit the initiator. This has the same effect as would pressing the initiator to, or beyond, the required resistance and stops the press when the predetermined load, which is greater than the expected load to produce the required resistance, is reached.

The above mentioned microswitches 17, 17a, 18 and 19 are optional and at least the two first mentioned would probably only be required when pressing explosive compositions. Since the circuitry involving these switches may take a number of different forms which would be readily apparent to any person skilled in the art, and they may be connected into a variety of convenient points ofthe servosystem, the switches have not been shown in any of the circuit diagrams illustrated herein.

The mould 13 shown in FIGURE 2 comprises a metal body 21 through which is formed a bore having a series of steps in its diameter. The two uppermost steps 22, 23 are designed to accommodate the body 24 and tail 25 of an igniter having the form shown and in which the body and tail are insulated from one another, and form the electrical contacts with the conductive filling. The step 22 forms a contact seating for the body -24 of the igniter while step 23 provides a clearance from the tail 25 which is thus insulated from the body 21 of the mould. The next lower step 26 is of considerably larger diameter and houses a liner 27 of insulating material. The final step constitutes a recess 28 of even greater diameter and houses a metal retaining ring 29, attached to the mould by screws 30, for securing the liner 27 in its bore. The igniter is normally provided, for safety in handling, With a shorting clip 31 which short circuits the body 24 to the tail 25 of the igniter and this clip is accommodated in a groove 34 formed on one side of the step 23 of the bore. A shallow recess in the upper face of the liner 27 accommodates an insulating washer 32 having on its upper surface, a hollow spigot 33 which extends into the step 23 of the bore around the tail 25 of the igniter and extends between the tail 25 and the clip 31 to break the short circuit during the pressing operation. This washer 32 is provided with a series of circumferentially spaced locating holes 35 any one of which can be located by a pin 36 carried by the liner 27 in order, in case of wear, to insert a new section of the spigot 33 between tail 25 and clip 31. An axial bore within the liner 27 accommodates at its upper end a plunger 37 having a spigot 38 which extends upward into the bore of the washer 32 to make contact with the tail 25 of the igniter. This plunger 37 is pressed upward by a spring 39 whose lower end rests on a plug 40 screwed into the liner 27 and having a head 41 which constitutes the external contact of an electrically conducting train comprising the tail 25 of the igniter, plunger 37,

spring 39 and plug 40. The upper end of the mould body I 21 carries a heavy, screw-on cap 42 having an axial bore 43 through which is inserted a drift 44 for compacting the composition in the igniter. The drift 44 may, as shown in FIGURE 2, be a separate element or may be permanently or replaceably attached to the ram 1 of the press.

Relative rotation between liner 27 and body 21 of the mould is prevented by the locating peg 45, and plug 40 is locked in position by set screw 46. It will be appreciated that the mould illustrated in FIGURE 2 is designed to accommodate a specific igniter and that variations in design, readily apparent to those skilled in the art, will be required in order to accommodate other forms of initiator.

The mould 13 is mounted, for pressing, in a shallow recess in the upper face of the base 4 of the press below the ram. As shown in FIGURE 3, electrical contact is made with the aforementioned train from igniter tail 25 to the head 41 of plug 40, by means of a spring loaded contact 47 mounted in and extending from the upper end of a metal casing 49, its spring 48 being retained in the casing 49 by a plug 50. The casing 49 is insulated from the base 4 of the press by a bush 51. Contact with the body of the igniter is made through the body 21 of the mould, to the base 4 of the press which is earthed. Leads 52, 53 complete the connection, by way of a cable such as is shown at 54 of FIGURE 1, to a comparator circuit to be later described.

The general arrangement of the servosystem is shown in block form in FIGURE 4. The igniter is connected into a comparator circuit, which is described in more detail with reference to FIGURES 5 and 6, and which is supplied with current from a suitable power source through a control unit. The output from the comparator circuit is fed to an amplifier whose output is used to control the current passing through the field windings of the electric motor 8. The armature of the motor 8 is supplied from a constant current supply. The motor whose speed is controlled by the amplifier output is used to drive the press as previously described.

Since the decrease in resistance of the initiator with increasing pressing load tends to be irregular and since this resistance, as will be described, is a controlling factor in respect of the motor speed, operation of the servosystem is stabilised by feeding back to the motor, through the amplifier, a voltage derived from a tachometer 15 (FIGS. 1 and 4) driven by the motor, which voltage is of course proportional to the motor speed.

In the comparator circuit shown in FIGURE 5 which is one circuit suitable for use in the servosystem of FIG- URE 4 the igniter is connected so as to form one arm of a resistance bridge circuit. An adjacent arm comprises a reference resistance R whose value is the predetermined value to which the igniter filling is to be pressed. The other two arms of the bridge are formed by equal resistances R R A suitable current supply is connected to the two opposite vertices 55, 56 of the bridge, which vertices constitute, respectively, the junctions between R and the igniter and between R and R The other two opposite vertices 57, 58, the latter of which is maintained at earth potential, are connected to the input of the amplifier. It will be clear that with this arrangement the input to the amplifier will vary from a maximum value when the igniter filling is uncompacted and its resistance, therefore, very high, to zero When the resistance of the igniter filling equals the reference resistance R In the alternative circuit shown in FIGURE 6 the igniter filling is connected, through the electrical contacts of the casing, in series with the reference resistance R and a suitable current supply, the reference resistance R being equal to the predetermined value to which the filling is to be pressed. Connections are made to each side of the igniter and reference resistance R the centre point 59 being at earth potential, the two voltages across these respective components being fed to a differential amplifier, the output from which is proportional to the difference between the two voltages.

It will be clear that using either of the two comparator circuits just described the output from the amplifier will vary from a maximum, when the filling of the igniter is uncompressed and its resistance, therefore, high, to zero when the igniter resistance is equal to the reference resistance R In each of the comparator circuits of FIGURE 5 and FIGURE 6, to provide a visual check on the progress of the pressing and to indicate that, on cessation of compaction, the igniter and reference resistances are, in fact equal, a micro-ammeter 60 in series with a high value adjustable resistance R4 is connected across the igniter, a balancing resistance R5 being connected across the reference resistance R The meter 60 is preferably arranged, by adjustment of the resistance R4, to read full scale before the igniter filling is pressed. It Will then read half scale when the igniter and reference resistance R are equal.

The speed of the motor of the servosystern will, of course, be proportional to the current passing through its field coils, which current is derived from the amplifier output. Thus, in operation, on initiating the pressing action through the control unit the motor begins to run and applies pressure to the igniter filling, whose resistance begins to decrease. The decrease in resistance brings it nearer to the resistance of the reference thus decreasing the motor current so that the motor slows down and finally stops when the initiator and reference resistances are equal. Application of a short circuit across the igniter, by way of the control unit will reverse the motor and release the load.

It will be appreciated that considerable variations in the circuitry can be made without departing from the scope of the invention. Similarly the invention is not confined to the type of press particularly described herein. The incorporation of the spring balance is desirable in the application described since, in addition to attaining the required resistance value, it is required that adequate consolidation be achieved. The balance will indicate that a required minimum load has been applied. For other applications it may be sufficient to use a simple damping spring or in some cases the spring may even be omitted altogether. It may further, in some instances, be possible to omit the tachometer feed back though it is preferred to include this stabilising device.

We claim:

1. A press for pressing compacts of electrically conductive compositions, which press incorporates a ram and a servosystem for controlling the movement of the ram in accordance with the electrical resistance of a compact being pressed; said servosystem incorporating a comparator circuit including a reference resistance, for comparing the resistance of the compact being pressed with the reference resistance; an amplifier to which is fed the output from the comparator circuit, for producing an amplifier output which decreases from a maximum when the compact is uncompressed and its resistance, therefore, high, to zero when the resistance of the compact equals the reference resistance; an electric motor to which is fed the amplifier output and whose speed is controlled thereby, said servosystem constituting means for slowing down the motor as the compact is compressed and for stopping the motor when the compact resistance and the reference resistance become equal; and drive means powered by said motor for operating the ram of the press.

2. A press as claimed in claim 1 wherein the comparator circuit comprises the compact, the reference resistance and a source of current connected in series, and circuit means for feeding the voltages across the compact and the reference resistance, respectively, to the amplifier; and wherein the amplifier is of the differential type.

3. A press as claimed in claim 2 incorporating indicator means for observing the progress of the pressing operation, which indicator means comprises a current meter and a high resistance, connected in series with one another and in parallel with the compact; and a balancing resistance connected in parallel With the reference resistance.

4. A press as claimed in claim -1 wherein the comparator circuit comprises the compact and the reference resistance connected to form, respectively, two adjacent arms of a resistance bridge; two fixed resistances of equal value connected to form the other two arms of said bridge; circuit means for feeding to the amplifier, the potential difference between that vertex of the bridge which constitutes the junction between the compact and the reference resistance, and that vertex which constitutes the junction between the two fixed resistances; and a current source connected to the remaining two vertices of the bridge.

5. A press as claimed in claim 4 incorporating indicator means for observing the pressing operation, which indicator means comprises a current meter and a high resistance connected in series with one another and in parallel with the compact; and a balancing resistance connected in parallel with the reference resistance.

6. A press as claimed in claim 1 incorporating a tachometer driven by the motor and producing a voltage proportional to the motor speed and circuit means for feeding back the tachometer voltage to the amplifier to stabilise the servosystern.

7. A press as claimed in claim 1 having a beam for operating the ram; a fixed member to which said beam is pivoted, the beam also being pivoted to the ram; an electric motor for driving the press and forming part of the servosystem; a winch driven by the electric motor; and a cord, connected to the beam and operatively connected to the winch for applying a load to the beam.

References Cited UNITED STATES PATENTS 2,949,391 8/ 1960 Anderson 148-9 2,455,823 12/ 1948 Tauber et a1. 318-488 2,602,294 7/1952 Sedgwick 218-34 2,809,395 10/1957 Gregory et al. 18-16 3,084,314 4/1963 Ziifer 318-20 3,100,858 4/1963 Topoazio et al. 318-48 3,123,754 3/1964 Longs et al 318-48 3,248,545 4/ 1966 Hansen. 3,260,937 7/1966 Ormand 318-20 3,260,938 7/1966 Ormand 318-20 FOREIGN PATENTS 941,445 11/ 1963 Great Britain.

ORIS L. RADER, Primary Examiner.

G. R. SIMMONS, Assistant Examiner. 

